The present invention relates to an electrically erasable non-volatile memory.
In recent years, it has been attempted to manufacture a recognition apparatus having a practical function such as pattern recognition after the model of a neural network of a living body. If such an apparatus is realized by an SiLSI which is currently highly integrated, a great merit is obtained. In this case, an element having the same function as that of a synapse which is a coupling portion between neurons must be developed. This function stores an analog coupling strength and can increase/decrease the stored coupling strength by learning. Although an electrically erasable programmable read only memory (to be abbreviated to an "EEPROM" hereinafter) is considered a predominant candidate for such an element, this EEPROM has the following drawbacks.
EEPROMs which are currently mainly used are classified into a floating electrode type as shown in FIG. 10 and an MNOS (Metal-Nitride-Oxide-Semiconductor) type as shown in FIG. 11. In FIGS. 10 and 11, reference numeral 1 denotes a control electrode; 2, a floating electrode; 3, a tunnel insulating film; 4, a source region; 5, a drain region; 6, an aluminum wire; 7, an Si substrate; 8, a gate; 9, an oxide film; 10, a nitride film; and T1, a power source terminal. An information write operation is realized by injecting an electric charge into the floating electrode or a trap in the interface between the oxide and nitride films by a tunnel current through the insulating film or channel hot electron injection.
In an element having either of the above structures, a charge injection rate largely depends on a potential difference between charge injection and storage sides. Therefore, when the potential of a charge storage portion changes as an electric charge is stored, it becomes difficult to continuously, linearly store the electric charge. This phenomenon will be described below by taking a case in which an electric charge is injected into a floating electrode by a tunnel effect as an example.
FIG. 12 shows a typical current-voltage characteristic obtained in a silicon oxide film (thickness=100.ANG., area=250.times.250 .mu.m) by a tunnel effect. When a current is small, a tunnel current is exponentially increased with respect to an applied voltage. In a general EEPROM, as shown in FIG. 10, a voltage is applied to the control electrode 1 formed above the floating electrode 2 and acts on the tunnel insulating film 3 by capacitive coupling. By this electric field, electrons are injected from the substrate 7 into the floating electrode 2 by the tunnel effect. When the electrons are stored in the floating electrode 2, the voltage applied on the tunnel insulating film 3 is reduced. As a result, the tunnel current is exponentially reduced in accordance with the characteristic shown in FIG. 12. For this reason, when a write operation is performed at a constant voltage, the potential of the floating electrode substantially logarithmically changes with respect to a time as shown in FIG. 13. In FIG. 13, the ordinate indicates a change amount of a threshold value of a MOS transistor having the floating electrode 2 as its gate, i.e., an amount proportional to a charge amount stored in the floating electrode 2.
For the above reason, it is difficult to write analog information in a conventional EEPROM, i.e., only digital information representing 1 or 0 (written or nonwritten) can be stored therein.
In order to store analog information in a conventional EEPROM, a write voltage value capable of injecting a charge amount corresponding to an analog amount to be written must be calculated and applied by an external computer, i.e., complicated control must be externally performed for an LSI. This is reported in, e.g., M. Holler, S. Tam, H. Castro, and R. Benson, "An Electrically Trainable Artificial Neural Network (ETANN) with 10240 "Floating Gate" Synapses," IJCNN (International Joint Conference on Neural Network), 1989, Vol. 2 of Articles, p. 191.